Electrolyte synthesis for ultracapacitors

ABSTRACT

A method of forming an electrolyte solution involves combining ammonium tetrafluoroborate and a quaternary ammonium halide in a liquid solvent to form a quaternary ammonium tetrafluoroborate and an ammonium halide. The ammonium halide precipitate is removed from the solvent to form an electrolyte solution. The reactants can be added step-wise to the solvent, and the method can include using a stoichiometric excess of the ammonium tetrafluoroborate to form a substantially halide ion-free electrolyte solution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No. 13/011,066 filed on Jan. 21, 2011, the content of which is relied upon and incorporated herein by reference in its entirety, and the benefit of priority under 35 U.S.C. §120 is hereby claimed.

BACKGROUND

The present disclosure relates generally to methods for forming electrolyte compositions, and more particularly to the synthesis of an electrolyte solution for use in ultracapacitors.

Energy storage devices such as ultracapacitors may be used in many applications where a discrete power pulse is required. Such applications range from cell phones to hybrid vehicles. An important characteristic of an ultracapacitor is the energy density that it can provide. The energy density of the device, which can comprise two or more carbon-based electrodes separated by a porous separator and/or an organic electrolyte, is largely determined by the properties of the electrolyte. A typical electrolyte utilized in commercial ultracapacitors comprises tetraethyl ammonium tetrafluoroborate (TEA-TFB) salt dissolved in a solvent such as acetonitrile. This electrolyte system has a number of beneficial properties, including salt solubility and ion conductivity.

One factor that is important in the development of electrolyte solutions is cost. Due to its relatively expensive synthesis and purification, commercially-available TEA-TFB is expensive. An example synthesis of TEA-TFB is disclosed in U.S. Pat. No. 5,705,696. The example process involves reacting tetraalkyl ammonium halides with metal tetrafluoroborates in an aqueous medium followed by membrane dialysis to remove metal halides. Another synthesis approach is disclosed in U.S. Pat. No. 7,641,807, which discloses combining a metal halide and a tetraalkyl halide in acetonitrile followed by filtering of the metal halide. The product of this process typically includes a high concentration of halide ions, such as chloride ions (e.g., 0.71 wt. % or 7100 ppm) as well as associated metal ions. Such a concentration of halide ions is understood to be detrimental to ultracapacitor performance.

In view of the foregoing, there is a need for a simple and economical synthesis process to produce high purity TEA-TFB salt and electrolyte solutions comprising TEA-TFB salt.

SUMMARY

A method of forming an electrolyte solution comprises combining ammonium tetrafluoroborate and a quaternary ammonium halide salt in a liquid solvent to form a quaternary ammonium tetrafluoroborate and an ammonium halide, and removing the ammonium halide from the solvent to form an electrolyte solution. The reaction can be carried out entirely at about room temperature. For instance, in an example embodiment the acts of combining and removing are performed at about 25° C. The combining can be performed under constant agitation. In embodiments, a stoichiometric excess of ammonium tetrafluoroborate can be used to minimize the concentration of halide ions in the product.

The resulting product is an electrolyte solution comprising a quaternary ammonium tetrafluoroborate salt dissolved in a solvent, wherein a concentration of chloride ions in the electrolyte solution is less than 1 ppm, a concentration of bromide ions in the electrolyte solution is less than 1000 ppm, a concentration of potassium ions in the electrolyte solution is less than 50 ppm, a concentration of sodium ions in the electrolyte solution is less than 50 ppm, a concentration of water in the electrolyte solution is less than 20 ppm, and/or a concentration of ammonium ions in the electrolyte solution is greater than 1 ppm.

Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a button cell according to one embodiment;

FIG. 2 is CV curve for an electrolyte solution prepared using a stoichiometric ratio of reactants; and

FIG. 3 is a CV curve for an electrolyte solution prepared using a stoichiometric excess of ammonium tetrafluoroborate.

DETAILED DESCRIPTION

A method of making quaternary ammonium tetrafluoroborate involves reacting one or more quaternary ammonium halides with ammonium tetrafluoroborate in an organic solvent. The reaction products are quaternary ammonium tetraflurorborate and ammonium bromide. The quaternary ammonium tetraflurorborate is soluble in the organic solvent, while the ammonium bromide forms as a precipitate. The precipitated NH₄Br can be filtered to form a solution of, for example, TEA-TFB in an organic solvent such as acetonitrile. In embodiments, the complete reaction is carried out at about room temperature under constant agitation.

In contrast to a number of known synthesis routes, which use metal tetrafluoroborates as reactants, the present method uses ammonium tetrafluoroborate as a reactant. While impurities derived from the conventionally-used metal compounds can contaminate the electrolyte and degrade device performance through Faradaic reactions, residual ammonium ions from the ammonium tetrafluoroborate reactant are not harmful to capacitor performance.

Suitable quaternary ammonium halides include tetramethyl ammonium bromide, tetraethyl ammonium bromide, tetrapropyl ammonium bromide, tetrabutyl ammonium bromide, triethyl methyl ammonium bromide, trimethyl ethyl ammonium bromide, and dimethyl diethyl ammonium bromide. Corresponding quaternary ammonium tetrafluoroborates include tetramethyl ammonium tetrafluoroborate, tetraethyl ammonium tetrafluoroborate, tetrapropyl ammonium tetrafluoroborate, tetrabutyl ammonium tetrafluoroborate, triethyl methyl ammonium tetrafluoroborate, trimethyl ethyl ammonium tetrafluoroborate, and dimethyl diethyl ammonium tetrafluoroborate.

In various embodiments, example organic solvents include dipolar aprotic solvents such as propylene carbonate, butylene carbonate, γ-butyrolactone, acetonitrile, propionitrile, and methoxyacetonitrile.

In embodiments, a quaternary ammonium halide can be combined with a stoichiometric excess of ammonium tetrafluoroborate. Thus, the electrolyte solution can be formed using a stoichiometric amount of ammonium tetrafluoroborate, or by using up to 150% (by mole) excess ammonium tetrafluoroborate. A molar ratio of quaternary ammonium halide to ammonium tetrafluoroborate can range from 1:1 to 1:1.5 (e.g., 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4 or 1:1.5). By using an excess of the ammonium tetrafluoroborate, the resulting solution can include an excess of BF₄ and NH₄ ions. Excess ammonium ions from the ammonium tetrafluoroborate can beneficially scavenge halide ions during the synthesis. Halide ions can also contribute to unwanted Faradaic reactions in the resulting electrolyte.

An electrolyte solution according to an embodiment comprises a quaternary ammonium tetrafluoroborate salt dissolved in a solvent, wherein a concentration of chloride ions in the electrolyte solution is less than 1 ppm, a concentration of bromide ions in the electrolyte solution is less than 1000 ppm (e.g., less than 800 or less than 700 ppm); a concentration of ammonium ions in the electrolyte solution is greater than 1 ppm, a concentration of potassium ions in the electrolyte solution is less than 50 ppm (e.g., less than 30 ppm), a concentration of sodium ions in the electrolyte solution is less than 50 ppm (e.g., less than 20 ppm), and/or a concentration of water in the electrolyte solution is less than 20 ppm (e.g., less than 10 ppm). A conductivity of the electrolyte solution at 25° C. can be at least 45 mS/cm (e.g., at least 45, 50, 55 or 60 mS/cm). A total concentration of the quaternary ammonium tetrafluoroborate salt in the electrolyte solution can range from 0.1M to 2M (e.g., 0.1, 0.2, 0.5, 1, 1.5 or 2M). The electrolyte solution can appear clear “water white” and have a density of about 0.877 g/ml.

The electrolyte solution can be stored, for example in a stainless steel drum, at room temperature under inert atmosphere (e.g., dry nitrogen) and under positive pressure.

Once formed, the electrolyte solution can be incorporated into an ultracapacitor. In a typical ultracapacitor, a pair of electrodes is separated by a porous separator and the electrode/separator/electrode stack is infiltrated with the electrolyte solution. The electrodes may comprise activated carbon that has optionally been mixed with other additives. The electrodes can be formed by compacting the electrode raw materials into a thin sheet that is laminated to a current collector via an optional conductive adhesion layer and an optional fused carbon layer. In addition to ultracapacitors such as electric double layer capacitors, the disclosed electrolytes can also be incorporated into other electrochemical electrode/device structures such as batteries or fuel cells.

Specific examples of activated carbon that may be used include coconut shell-based activated carbon, petroleum coke-based activated carbon, pitch-based activated carbon, polyvinylidene chloride-based activated carbon, polyacene-based activated carbon, phenolic resin-based activated carbon, polyacrylonitrile-based activated carbon, and activated carbon from natural sources such as coal, charcoal or other natural organic sources. Various aspects of suitable porous or activated carbon materials are disclosed in commonly-owned U.S. patent application Ser. Nos. 12/970,028 and 12/970,073, the entire contents of which are incorporated herein by reference.

Activated carbon can be characterized by a high surface area. High surface area electrodes can enable high energy density devices. By high surface area activated carbon is meant an activated carbon having a surface area of at least 100 m²/g (e.g., at least 100, 500, 1000 or 1500 m²/g).

The electrodes used to form an ultracapacitor can be configured identically or differently from one another. In embodiments, at least one electrode comprises activated carbon. An electrode that includes a majority by weight of activated carbon is referred to herein as an activated carbon electrode. In embodiments, an activated carbon electrode includes greater that about 50 wt. % activated carbon (e.g., at least 50, 60, 70, 80, 90 or 95 wt. % activated carbon).

In embodiments, the activated carbon comprises pores having a size of ≦1 nm, which provide a combined pore volume of ≧0.3 cm³/g; pores having a size of from >1 nm to ≦2 nm, which provide a combined pore volume of ≧0.05 cm³/g; and <0.15 cm³/g combined pore volume of any pores having a size of >2 nm.

In addition to activated carbon, additives such as binders and conductivity promoters can be used to control the properties of the electrode. Electrodes can include one or more binders. Binders can function to provide mechanical stability to an electrode by promoting cohesion in loosely assembled particulate materials. Binders can include polymers, co-polymers, or similar high molecular weight substances capable of binding the activated carbon (and other optional components) together to form porous structures. Specific exemplary binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride, or other fluoropolymer particles; thermoplastic resins such as polypropylene, polyethylene, or others; rubber-based binders such as styrene-butadiene rubber (SBR); and combinations thereof. In embodiments, PTFE can be utilized as a binder. In further embodiments, fibrillated PTFE can be utilized as a binder. By way of example, an electrode can include up to about 20 wt % of binder (e.g., up to about 5, 10, 15, or 20 wt %).

An electrode can also include one or more conductivity promoters. A conductivity promoter functions to increase the overall conductivity of the electrode. Exemplary conductivity promoters include carbon black, natural graphite, artificial graphite, graphitic carbon, carbon nanotubes or nanowires, metal fibers or nanowires, graphenes, and combinations thereof. In embodiments, carbon black can be used as a conductivity promoter. In embodiments, an electrode can include up to about 10 wt % of a conductivity promoter. For example, an electrode can include from about 1 wt % to about 10 wt % of conductivity promoter (e.g., 1, 2, 4, or 10 wt %).

Example ultracapacitors can include one activated carbon electrode or two activated carbon electrodes. For example, one electrode can include a majority of activated carbon and the other electrode can include a majority of graphite.

The electrolyte solution can be characterized by measurements performed on the electrolyte solution itself, as well as by measurements performed on test cells that incorporate the electrolyte solution.

An embodiment of an EDLC, a button cell, is shown in FIG. 1. The button cell 10 includes two current collectors 12, two sealing members 14, two electrodes 16, a separator 18, and an electrolyte solution 20. Two electrodes 16, each having a sealing member 14 disposed around the periphery of the electrode, are disposed such that the electrode 16 maintains contact with a current collector 12. A separator 18 is disposed between the two electrodes 16. An electrolyte solution 20 is contained between the two sealing members.

An activated carbon-based electrode having a thickness in the range of about 50-300 micrometers can be prepared by rolling and pressing a powder mixture comprising 80-90 wt. % microporous activated carbon, 0-10 wt. % carbon black and 5-20 wt. % binder (e.g., a fluorocarbon binder such as PTFE or PVDF). Optionally, a liquid can be used to form the powder mixture into a paste that can be pressed into a sheet and dried. Activated carbon-containing sheets can be calendared, stamped or otherwise patterned and laminated to a conductive adhesion layer to form an electrode.

The button cells were fabricated using activated carbon electrodes The activated carbon electrodes were fabricated by first mixing activated carbon with carbon black in an 85:5 ratio. PTFE was added to make a 85:5:10 ratio of carbon:carbon black:PTFE. The powder mixture was added to isopropyl alcohol, mixed, and then dried. The dried material was pressed into a 10 mil thick pre-form. The pre-forms were then laminated over a conductive adhesion layer (50 wt. % graphite, 50 wt. % carbon black), which was formed over a fused carbon-coated current collector.

For the button cells, the current collectors were formed from platinum foil, and the separator was formed from cellulose paper. Prior to assembly, the activated carbon electrodes and the separator were soaked in an electrolyte. A thermoset polymer ring is formed around the periphery of the assembly to seal the cell, which is filled with an organic electrolyte such as tetraethylammonium-tetrafluoroborate (TEA-TFB) in acetonitrile. Prior to sealing the cell, an extra drop of the electrolyte was added to the cell.

Electrochemical experiments were used to test the cell, included cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge. Cyclic voltammetry experiments were performed at a scan rate of 20 mV/sec within various potential windows over the maximum range of 0 to 4.5 V. The EIS test included measuring impedance while applying an AC perturbation with an amplitude of 10 mV at a constant DC voltage of 0 V over the frequency range of 0.01-10,000 Hz. Galvanostatic charge/discharge experiments were performed at a current magnitude of 10 mA.

The energy density of the device was calculated using the Integrated Energy Method. The galvanostatic data (potential vs. time data) was numerically integrated and multiplied by the discharge current to obtain the energy delivered by the device (in Ws) between two potentials V₁ and V₂.

E = I_(disch) * ∫_(V 1)^(V 2)V𝕕t

The device capacitance (C_(device) in Farads) can be calculated from the energy according to the following relationship:

$C_{device} = \frac{2E}{\left( {V_{1\;}^{2} - V_{2}^{2}} \right)}$

The specific capacitance (F/cm³) was then calculated by dividing the device capacitance by the total volume of the carbon electrodes.

The stable voltage, which is the maximum voltage the device can withstand without appreciable Faradaic reactions, was measured from a series of cyclic voltammetry (CV) experiments performed over several different voltage windows. From the CV data, a Faradaic Fraction was measured using the following equation:

${{Faradaic}\mspace{14mu}{Fraction}} = {\frac{Q_{Faradaic}}{Q_{{non}\text{-}{Faradaic}}} = \frac{\left( {Q_{anodic} - Q_{cathodic}} \right)}{Q_{cathodic}}}$

The charge (Q) during anodic and cathodic scans was calculated by integrating the CV curve and dividing the result by the scan rate at which the CV was performed. The stable voltage was defined as the potential at which the Faradaic Fraction is approximately 0.1.

The energy density at the stable voltage, which is the maximum voltage the device can withstand without appreciable Faradaic reactions, was calculated using the following relation where C_(device) is the device capacitance (in Farads), V₁ is the stable voltage, V₂ is V₁/2, and Volume is the device volume in liters:

${{Energy}\mspace{14mu}{density}\mspace{14mu}\left( {{Wh}\text{/}L} \right)} = {\frac{1}{2}{C_{device}\left( {V_{1}^{2} - V_{2}^{2}} \right)}\frac{3600}{Volume}}$

Additional aspects of the disclosure are set forth in the following non-limiting examples, which disclose the example synthesis of TEA-TFB in acetonitrile from ammonium tetrafluoroborate and tetraethyl ammonium bromide.

EXAMPLE 1

In 100 ml of acetonitrile, 31.3329 g of tetraethyl ammonium bromide (TEA-Br) was added and the suspension was stirred for 1 hr followed by the addition of 15.642 g of ammonium tetrafluoroborate (NH₄BF₄). The amount of reactants corresponds to a stoichiometric amount. The suspension was stirred, and the temperature of the mixture was maintained at 25° C. throughout the synthesis.

The suspension was filtered to remove the precipitate. The conductivity of the electrolyte solution was 64 mS/cm. The resulting electrolyte solution was incorporated into a button cell as described above using activated carbon having a surface area of 1800 m²/g.

The energy density of the button cell was 15 Wh/1. Referring to FIG. 2, however, significant Faradaic reactions are seen with the electrolyte. The bromide ion content in the electrolyte solution determined by ion chromatography was 7123 ppm. The bromide ions cause Faradaic reactions and, together with other halide ions, undesirably increase the cell's ESR and reduce cycle life.

EXAMPLE 2

In 100 ml of acetonitrile, 31.3329 g tetraethyl ammonium bromide was added and the suspension was stirred for 1 hr followed by the addition of 25.642 g ammonium tetrafluoroborate. The amount of reactants corresponds to a stoichiometric excess of ammonium tetrafluoroborate. The suspension was stirred, and as with Example 1, the temperature was maintained at 25° C.

The suspension was filtered to remove the precipitate. The conductivity of the electrolyte solution was 64 mS/cm. The resulting electrolyte solution was incorporated into a button cell as described above using activated carbon having a surface area of 1800 m²/g.

The energy density of the button cell was 17 Wh/1. Referring to FIG. 3, the CV curve showed no Faradaic reactions. The bromide ion content in the electrolyte solution determined by ion chromatography data was 751 ppm. The chloride ion content was less than 0.05 ppm, and concentration of ammonium ions was 245 ppm.

EXAMPLE 3

An electrolyte solution having the same overall amount of reactants as Example 2 was prepared via the step-wise addition of reactants. As defined herein, a step-wise addition of reactants means that at least one (preferably both) of the reactants is introduced to the mixture both before and after the introduction of the other reactant. Thus, a step-wise addition of reactants A and B can include the introduction of the reactants in the following example sequences: ABA, BAB, ABAB, BABA, ABABA, BABAB, etc.

In 100 ml of acetonitrile, under constant stirring at 25° C. with 1 hr periods between reactant additions, the following were added in sequence: 5 g NH₄BF₄, 10 g TEA-Br, 5 g NH₄BF₄, 10 g TEA-Br, 5.642 g NH₄BF₄, 11.332 g TEA-Br, and 10 g NH₄BF₄. After the final addition, the solution was stirred overnight and then filtered with Whitman 42, 110 mm paper, and then filtered again with 0.02 um syringe filter Filtered again with 0.02 micron syringe filter.

As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a “metal” includes examples having two or more such “metals” unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, examples include from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred.

It is also noted that recitations herein refer to a component of the present invention being “configured” or “adapted to” function in a particular way. In this respect, such a component is “configured” or “adapted to” embody a particular property, or function in a particular manner, where such recitations are structural recitations as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is “configured” or “adapted to” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component.

It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Since modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A method of forming an electrolyte solution comprising: combining ammonium tetrafluoroborate and tetraethyl ammonium bromide as reactants to form a suspension in a liquid solvent; reacting the ammonium tetrafluoroborate and the tetraethyl ammonium bromide to form tetraethyl ammonium tetrafluoroborate and ammonium bromide; and removing the ammonium bromide from the solvent to form an electrolyte solution, wherein the amount of at least one of the reactants is greater than that reactant's solubility in the solvent.
 2. The method according to claim 1, wherein a molar ratio of ammonium tetrafluoroborate to quaternary ammonium halide is from 1:1 to 1.5:1.
 3. The method according to claim 1, wherein the solvent is selected from the group consisting of propylene carbonate, butylene carbonate, γ-butyrolactone, acetonitrile, propionitrile, and methoxyacetonitrile.
 4. The method according to claim 1, wherein the combining and the removing are performed at about 25° C.
 5. The method according to claim 1, wherein the combining is performed under constant agitation.
 6. The method according to claim 1, wherein the combining comprises a step-wise addition of the ammonium tetrafluoroborate and the tetraethyl ammonium bromide to the liquid solvent.
 7. The method according to claim 1, wherein a concentration of the quaternary ammonium tetrafluoroborate in the solvent is from 0.1 to 2 molar.
 8. The method according to claim 1, wherein a conductivity of the electrolyte solution at 25° C. is at least 45 mS/cm.
 9. The method according to claim 1, wherein a concentration of chloride ions in the electrolyte solution is less than 1 ppm; a concentration of bromide ions in the electrolyte solution is less than 1000 ppm; and a concentration of ammonium ions in the electrolyte solution is greater than 1 ppm.
 10. The method according to claim 1, wherein a concentration of potassium ions in the electrolyte solution is less than 50 ppm; a concentration of sodium ions in the electrolyte solution is less than 50 ppm; and a concentration of water in the electrolyte solution is less than 20 ppm.
 11. The method according to claim 1, wherein a concentration of the quaternary ammonium tetrafluoroborate in the solvent is from 0.5 to 2 molar. 